3 Replies Latest reply on Jan 19, 2015 7:22 AM by Dave Laban

    More information required?

    Derek Westwood

      A friend asked me to help her with a coursework question, I have pasted it below, I think a lot more information is required to even model this before working out freedoms. Am I missing something?

       

      "A climbing stool in a library is used for reaching books on high shelves. It consists of a short cylinder on which a person can stand, mounted on 3 spherical bearings that touch the floor. By considering the constraints on the stools motion, determine how many freedoms the stool has, and describe how it can move."

       

      Regards, Derek.

        • Re: More information required?
          Dave Laban

          Kind of depends on what level of education this is for.

           

          It's free to move in translation in two directions (assuming gravity is functioning correctly it won't move anywhere vertically).  It's also free to rotate about the centerline of the short cylinder.

           

          If you were being fussy you could say it can also rotate about the three axes between the three pairs of spherical bearings but that might be over thinking it.

           

          So effectively free motion in X, Y and RZ.  No motion in Z.  Theoretically no motion about RX and RY.

            • Re: More information required?
              Derek Westwood

              How are you mounting the spherical bearings? and why 3 pairs?

              Thanks, Derek

                • Re: More information required?
                  Dave Laban

                  Is the specific mounting relevant to the number of DOF?  As a library stool I've inferred you don't want vertical movement and by the question specifying spherical, you'll get free translation on the floor.

                   

                  Consider the three bearings (presumably spaced as an equilateral triangle) within the circle defined by the stool named as A, B and C.  There will be a straight line between AB, BC and CA.  If you then put the weight of a person in the area of the stool subtended by one of these lines and the OD of the circle, you'd then be able to tip the stool on axis AB (for example) until the bottom rim of the stool contacted the floor (assuming for practicalities sake there is a clearance gap between the base of the stool and the floor).